Tape tension motor control circuit



April 5, 1966 F. s. c. BRANco 3,244,954

TAPE TENSION MOTOR CONTROL CIRCUIT Filed Jan. .'50, 1962 2 Sheets-Sheetl 1.50 J0 /jf /JZ 15 jm, imp

April 5, 1966 F. s. c. BRANco TAPE TENSION MOTOR CONTROL CIRCUIT 2Sheets-Sheet 2 Filed Jan. 30, 1962 NN w United States Patent Thisinvention relates to an electrical system for regulating the tension ina movable medium, such as arta'pe.

In recent years, information has been recorded on tape for stroage andfor subsequent reproduction from the tape. For example, video and audioinformation has been recordedon tapes for subsequent use. informationsuch as digital data obtained from computers and other scientificinstruments has also been' recorded on' the tape for subsequent use.

In order to accurately record information on the medium such as a tapeand reproduce information from the tape, the tension in the tape must bekept substantially constant. If the tension varies during portions ofthe recording or reproducing cycles, the tape medium may stretch on thetape reel. When the tape remains in this streached condition, itlbecomes permanently deformed and information on the tape is distoied;Also, asA thetension varies, the instantaneous speed of the tapemediumtvaries to cause flutter in the system. Itis, therefore, importantto keep the tension on the tape at a constant` value.

This has been accomplished' with fair success in the past` by using arst motor to drive the take-up reel receiving the tape at a speedV tomaintain the tension in the tape mediumv at a constant value. A secondmotor driving the pay-out reel is also Varied in itsy speed to maintainthe tension in the tape at thevpay-out rcel at a constant value. In thismanner, both motors are varied independently to control the tension inthe tape medium at` their respective reels.

There are many variable factors which cause the tension in the tape tochange at different instants of time from a particular value. Forexample, inaccuraciesin thel structure of thepay-out and take-up reelsmay affect the tape tension; As another eXaniple,`the voltagesapplied`to the motors can vary so as to produce corresponding The abovevariablefactors have created a need for a` control system which is accurate,simple and fast acting. The prior art in responding to this need hasdeveloped fairly successful systems, but has not been able tocornpletely solve the problem. For example, the prior art has utilizedysystems which producera control signal having an amplituderepresentative of the magnitude of the error in tape tension from aparticularvalue and having a polarity representative of the direction ofdeviation of tape tension from the particular value.v The polarity ofthe signal is used to control the direction in which the motor coupledto the reel is operated. The amplitude of the signal ldetermines thespeed at which the motor coupled to the reel is operated.

Since the prior art system utilize control signals of both polarities,circuitry must be provided for the signals of both polarities. Thisresultsin two parallel control channels, and in order for the controlsystem to operate properly, the channels must be identical; Furthermore,supply voltages of one polarity must be introduced to one of thechannelsand supply voltages of opposite polarity but of equal magnitudemust be introduced to the other channel'. If one channel is not balancedwith respect to the other channel, either because of imbalances in itsown constructiion or imbalances in the supply voltages, signals. ofdifferent amplitudes for comparable tensions onboth sides of the desiredtape tension is produced. This creates an' inaccuracy and` leads to thecontrol system hunting back and forth in an effort to nd the propertension.

This invention provides' control signals all of the same polarity toeliminate the need' for balanced circuitry and to eliminate the need forsupply voltages of opposite polaritiesbut of equal magnitudes. Theinvention allows the use of relatively simple circuitry since there isnow no need for components for both positive and negative signals. Theinvention is accomplished by producing control signals having the samepolarity whether the tension on the tape is above or below theparticular value. This is accomplished by biasing the system so thatsignals of a` first polarity but'ofhan increasing amplitude are producedas the tension on the tape deviates in one direction from theparticular` value. Similarly, signals of thefirst polarity *but of adecreasingu amplitude are produced as the tension in the tape deviatesVin the opposite direction from the particular value. In this way,amplitudes of the signal above a particular null value indicatedeviations in the tension on the tape in one direction and amplitudes ofthe signal below the particular value indicate deviations in the tensionon the tape in-the opposite direction. Signals on onel side of theparticular null value cause a reversirigswitch t'o` operate in a firststate and signals on the other side of the particular null value causethe reversing switchV to operatel in a second state. The reversingswitch controls the polarity of the signals appliedy to the motordriving the pay-out reel so as to control the direction in which themotor is operated.

Since biasing the control signals provides lower amplitude signals forcomparative tensions'on one side of the desired tension than on theother, a diiferential amplier is provided to transpose one side of thecontrol signal. Because of the operation of the differential amplifier,the amplitude' of the control signal has a V-shaped characteristic atthe different tensions in the tape with the vertex of the V occurring atthe'particular tensiony in the tape. The V-shaped. characteristiccausesl the amplitude of the signal to be the same for correspondingdeviations in the tension in the tape above and below the' particularvalue. The transposed control signals are applied to the motor tocontrol the speed of the motor, the polarity of the motor beingdependent upon the operation of the reversing switch in the first andsecond states. The motor operates to drive the reel in a direction toaid the movement of the tape for tensions in the tape above theparticular value and to oppose the movements of the tape for tensions inthe tape below the particular value.

In the drawings:

FIGURE l is a somewhat schematic view of a system for driving a mediumsuch as a tape from a pay-out reel to a take-up reel;

FIGURE 2 is a schematic view, substantially in blockV form, of oneembodiment of a system constituting this invention for controlling thetension at which the tape is maintained at each instant, at either thepay-out reel or the take-up reel, curves showing voltage waveformsproduced by the different stages also being included in the figure; g

FIGURE 3 is an enlarged schematic illustation of a differentialtransformer included in the embodiment shown in FIGURE 2;

FIGURE 4 is a detailed circuit diagram of the embodiment shown inFIGURES 2 and 3;

FIGURE 5 is a schematic diagram, substantially in block form, of asecond embodiment of the system constituting this invention;

FIGURE 5 is a schematic diagram of photoelectric apparatus included inthe embodiment shown in FIGURE 5;

FIGURE 7 is a somewhat schematic view of an electrical circuit forenergizing the photoelectric apparatus shown in FIGURE 6; and

FIGURE 8 is a schematic view of a vane included in the photoelectricapparatus shown in FIGURE 6.

FIGURE l illustrates a tape drive system for recording information on atape medium and for subsequently reproducing the information from thetape medium. In the apparatus shown in FIGURE l, a tape 300 is driven inthe direction shown by the arows so as to become unwoun-d from a pay-outreel 302 and become wound on a take-up reel 303. The pay-out reel 302 isdriven by a motor 304 and the take-up reel 303 is driven by a motor 305.The pay-out reel 302 and the take-up reel 303 are driven oy the motors304 and 305, in either one of two opposite directions dependent upon thepolarity of the voltage applied to the motors.

For example, if the tension in the tape 300 is above the particularvalue desired, the motor 304 rotates in a direction to aid the movementof the tape. This relieves tension on the tape 300 and allows thetension in the tape to decrease quickly to the desired value. The motor304 is excited in an opposite direction when the tape tension is belowthe desired value. This imposes additional tension on the tape 300 so asto increase the tension in the tape to the desired value. The motor 304still rotates in the direction of tape travel but it applies torque inthe opposite direction. The motor 305 is controlled in a similar mannerto maintain the tension in the tape at the take-up reel at a constantvalue.

As the tape 300 unwinds from the pay-out reel 302, it passes over aguide roller 306 which directs the tape to pass over a guide roller 308.The guide roller 308 is controllably tensioned by a spring 310 to biasthe guide roller in a direction opposite to the direction in which themoving tape biases the guide roller 30S. The guide roller 308 is in acentral position when the tape tension has the desired value since thetension produced by the spring 310 is adjusted to equal the tensionproduced by the moving tape. The gui-de roller 30S swings in either oftwo directions, as indicated by arrows 311, when the tension produced bythe moving tape is greater or less than the tension of the spring.

The tape 300 after passing over guide roller 308 makes a turn of 90around guide roller 314. The tape now is in position to pass by theheads 316 and makes another turn of 90 between capstan 320 and pressureroller 321. The pressure roller 321 is engaged when the tape system isin operation. The tape passes over guide roller 308 which iscontrollably tensioned by a spring 310 in the same manner as guideroller 368 and spring 310. The tape is directed to guide roller 32,2which passes the tape to take-up reel 303.

Connected to the guide rollers 308 and 308 are arms 312 and 312' whichhave vanes 21 and 21 positioned at the end of the arms. The vanes 21 and21 are used in individual differential transformers, one ot which isgenerally indicated at 11 in FIGURE 3, to follow the movement of theguide rollers 308 and 308 and to control the production of signals bythe differential transformer in accordance with such movement.

The differential transformer 11 includes three windings on a commonE-shaped core 20. A primary winding 1-2 is wound on the center leg ofthe core 20, with two secondary windings 3J@ and S-6 on the two outerlegs of the core 20. The secondary windings are wound with an oppositesense, with terminals 3 and 5 connected together, and with the outputsignal produced across terminals 4 andv 6. The vane 21 is made ofmagnetic material and is used to couple the primary winding 1-2 to thesecondary windings 3-4 and 5-6. The positon of the vane determines whichof the two secondary windings is more closely coupled to the primarywinding or if the secondary windings are equally coupled to the primarywinding.

Since the position of the vane 21 is determined by the tension in thetape, the transformer 11 is adjusted to produce a zero output signalwhen the tape tension is at the desired value. This is accomplished byhaving the vane 21 in a central position at the desired tension in thetape 300 to couple the primary winding equally to each of the secondarywindings. Since the secondary windings are wound in an opposite sense,the signals produced in the secondary windings are of equal amplitudewith opposite phase to cancel each other.

When the tension in the tape changes from the desired value, the vane 21rotates and the primary winding is coupled unequally to the secondarywindings. One or the other of the secondary windings will predominate toproduce an output signal. The amplitude of the output signal from thedifferential transformer is dependent upon the tension in the tape, andthe phase of the output signal is dependent upon whether the tension inthe tape is higher or lower than the desired value.

FIGURE 2 illustrates in block f-orm a system for controlling the tensionin a medium such as a tape according to the concepts of this invention.In the block diagram of FIGURE 2, an oscillator 10 produces an outputsignal which is introduced to the differential transformer 11. Asillustrated in FIGURE 3, the differential transformer 11 is controlled,for example, by the vane 21 which monitors the tension in the tapemedium at the pay-out reel as described above to produce signal 110.When the tape tension passes from below the desired tension to above thedesired tension, point 111 is produced which indicates a change in phaseof the signal 110. The point 112 signifies the tape tension returning toits former position below the desi-red tension. l

The signal then passes t-o an A.C. amplifier 12 to obtain anamplification of the signal. Incorporated in the amplifier 12 is a D.C.bias to produce a signal 120 which has a single polarity regardless ofthe deviations in the tension in the tape from a particular value.Points 121 and 122 are similar to points 111 and 112 and indicate achange in phase of the signal 120.

Phase detector 13 receives the signal from the amplifier 12 and comparesit with the signal from the oscillator 1t) to determine if the signalsare in phase or if they are 180 out of phase. The phase detector 13 isconstructed to produce a direct voltage having a magnitude directlydependent upon the tension of the tape. The phase detector 13 is biasedto produce a direct voltage having a single polarity regardless ofdeviations in the tension in the tape above or below a desired value.Point 131 corresponds to the value of the bias in the phase detector 13.Signals above the point 131 are representative of signals of the samephase from the oscillator 10 and the differential transformer 12, and,therefore, are yrepresentative of tape tensions to one side of thedesired value. Signals below the point 131 are representative of signalsof opposite phase from l@he oscillator 10 and the differentialtransformer 12 and, therefore, are representative of tape tensions tothe other side of the desired value.

The signal is applied to both the differential ampli` yfier 14 and therelay amplifier 16. The differential amplifir 14 transposes the variousvalues tof the signal 130 below the point 131 to corresponding positionsabove the point 131 to produce a V-shaped response as `shown by signal140. The vertex point 141 of the signal 140 cor-` responds to the point131 of the signal 130. In this way, the differential amplifier 14effectively produces signals of equal amplitude for correspondingdeviations in the tension in the tape medium on opposite sides of thedesired value. The signal 140 then passes through the D.C. `amplif fier14 which lamplifies the signal and biases the signal to have the vertexlocated at a reference potential. This is shown as a signal 150 which isthen applied to a reversing relay 17.

The reversing relay 17 is controlled by the magnitude of signal 130. Therelay is designed to have a first state of oper-ation for signals abovethe point 131 land a second state of operation for signals below thepoint 131. This results from the operation of a relay amplifier 16 whichis biased to a state of conductivity for signals on one side of thepoint 131 and which becomes non-conductive for signals on the other sideof the point 131. Since the point 131 occurs at the `same tape tension.as the point 151 of the signal 150, signals to the left of point 151pass through the reversing relay 17 undistubed, while signals to theright of the point 151 have their polarity reversed by the reversingrelay 17. This results in the application of a control signal 170 to themotor 304-. The solid line is the resultant `of the signals to the leftof point 151, and the dotted line of reverse polarity is the resultantof the signals to the right of point 151 as reversed by the reversingrelay 17. The .application of the control signal 170 to the motor causesthe motor to drive lthe pay-out reel in a direction and at a speed ateach instant to maintain the tension in the tape constant at the desiredvalue. The system of FIGURE 2 can be used in a similar manner to controlthe tape tension at the take-up reel.

FIGURE 4 illustrates in detail a system for performing the functionsshown in FIGURE 2. The system includes the oscillator 10, which mayoperate at a suitable frequency such as 40 kilocycles per second.Included in the oscillator 1t) is a current control member such as atransistor 30, which may be `a PNP type 2N597. Connected between theemitter of transistor an-d a reference potential such as ground are aresistance 31 and a capacitance 32. The resistance 31 may have a valueof 220 ohms and the capacitance 32 may have a value of l microfarad.Winding 1 2 of a transformer 33 is connected between the base oftransistor 3@ and the junction of the resistances 34 and 35. Theresistances 34 and 35 may have val-ues of 10 and 100 kilo-hmsrespectively. The resistance 34 is connected at its other terminal tothe 4reference potential such as ground, and the resistance 35 isconnected to the junction of a capacitance 36, which may have a value of50 mircofarads, and a resistance 37, which may have a value of 27 ohms.The other terminals of capacitance 36 and resistance 37 -arerespectively connected to ground and soruce V1. Source V1 may have avalue of 18 volts.

Also connected to the junction of the capacitance 36 and the resistance37 are winding 3 4 of transformer 33 and capacitance 38. The capacitance38 may have a value of 0.047 microfarad. The other terminals of winding3 4 and capacitance 38 are connected to the collector of the transistor30. Output windings 5 6 and '7 8 of transformer 33 supply other portionsof the system with signals from the oscillator.

The oscillator is of the tuned-collector type. The tuned circuitconsists of winding 3 4 of transformer 33 and capacitance 38. Theresistors 34 and 35 establish the base bias. The resistor 31 is theemitter bias stabilizing resistance. The capacitance 32 by-passesalternating curtrent around resistor 31 and the capacitance 36 by-passesalternating current around resistors 34 and 35i. Oscillations will startupon the application of a direct volt-age through dropping resistor 37,and regeneration is accomplished by coupling the feedback signal fromthe winding 3 4 of transformer 33 to the winding 1 2 of transformer 33.

The output Winding 7 8 of transformer 33 is connected to the primarywinding 1 2 of a differential transformer 11. The terminal 3 of thewinding 3 4 in the transformer `11 is connected to the `referencepotential such as ground, and the terminal 6 of the winding 5 6 in thetransformer is connected to the junction of a resistance 40 and acapacitance 41. The resistance 40 may have a value of ohms and thecapacitance 41 may be 0.01 microfarad. As previously described, theoutput signal across the terminals 4 and 6 of the transformer 11 iseither in phase or out of phase with the input signal to the prima-rywinding of the transformer, depending upon the direction of deviation oftape tension from the desired value. The output signal from thesecondary windings 3 4 and 5 6 of the differential transformer 11 isapplied across resistance 40 and through the coupling capacitor 41 tothe base of a transistor 42.

The transistor 42 can also be a PNP type 2N597. A resistor 43 which mayhave a value of 100 kilohms is connected between the base of thetransistor 42 and the source V1. A resistor 44 which may have a value of10 kilohms is connected between the base of the transistor 42 and thereference potential such as ground. The emitter of transistor 42 isconnected to one end terminal of a potentiometer 45, the other endterminal of which is at the reference potential such as ground. Thepotentiometer 45 may have a Value of l kilohm from one end terminal tothe other end terminal. The movable arm of the potentiometer 45 iscoupled electrically 4for alternating signals through capacitor 45 toground. The capacitor 46 may have a value of l microfarad. The collectorof the transistor 42 is connected to the source V1 `and also to oneterminal of a capacitor 39 having a value of 0.1 microfarad. The otherterminal of the capacitor 39 is connected to one end terminal of aresistor 47 which may have a value of 10 kilohms. The other terminal ofresistor 47 has a common connection with the movable arm of apotentiometer 48. The end terminals of the potentiometer 48 arerespectively con nected to the source V1 and the reference potentialsuch as ground. The potentiometer 48 may have a value of 5 kilohoms fromone end terminal to the other end terminal.

The transistor 42 `and the associated circuitry form an amplifier forthe alternating signals from the differential transformer 11 and alsoprovide a DC. bias for the incoming signals. Resistor 44 provides a baseDC. return path. The potentiometer 45 provides bias stability and givesoptimum response and stability in the amplifier circuit. Capacitor 46bypasses the A.C. signal around the lower portion of the potentiometer45. The resistance 43 is the collector load resistor, and thealternating signals constituting the output fromr the amplifier arecoupled through the blocking capacitor 39, which also blocks the passageof any direct voltage.

The resistor 47 and the potentiometer 4S provide a D.C. bias on theoutput signals from the amplifier so that the output signals are all ofthe same polarity. The jumper 65 is opened and the arm of potentiometer4S is adjusted to produce a level corresponding to the level 131 in FIG.2 when the differential transformer 11 provides a null outputcorresponding to the point 111 in FIG. 2. This gives the correct bias tothe amplifier output signals to indicate that the tape tension is at theesired value.

The biased output signal from the junction of the capacitor 39 and theresistor 47 is introduced to the junction of diodes 49 and 50. Thediodes 49 and 50 form a phase detector with diodes 51 and 52. They canall be type 1N99. The cathode of the diode 49 is connected to the anodeof the diode 50. The cathode of diode 51 has a common connection withthe anode of the diode 52. The anodes of the diodes 49 and 51 areconnected to the terminal 5 of the winding 5 6 in the transformer 33.The cathodes of the diodes 50 and 52 are coupled electrically throughthe parallel combination of a capacitor 53 an-d a resistor 54 to theterminal o of the winding 5 6 of transformer 33. The capacitor 53 mayhave a 7 value of 0.1 microfarad and the resistor 54 may have a value of100 kilohms.

The phase detector compares the signal across the junction of the diodes49 and 51 and the junction of the diodes S and 52 with the signal at thejunction of the diodes 49 and 50 to determine if the signals are inphase or 180 out of phase. The signal introduced from the oscillatoracross the junction of the diodes 49 and 51 and the junction of thediodes 50 and 52 has a constant phase. However, the signal passing tothe junction of the diodes 49 and 50 from the amplifier 12 can be eitherin phase with the oscillator signal or 180 out of phase with theoscillator signal depending upon the position of the vane 21 in thedifferential transformer 11. T he output of the phase detector indicatesif the tension in the tape medium is above or below the desired value.The output is taken from the junction of the diodes S1 and 52 and isapplied to a capacitor 55, which is connected between the outputjunction and ground and may have a value of 0.01 microfarad.

The diodes 49, 50, 51 and 52 .are forward biased during the introductionof positive signals on the anodes of the diodes 49 and 51 relative tothe signals on the cathodes of diodes S0 and 52. The impedances of thediodes will then all be relatively low and the voltage drop across thediodes will be negligible. The voltage introduced to the cathode of thediode 49 and the anode of the diode 50 then appears at the junction ofthe cathode of the diode 51 and the anode of the diode 52. When thediodes 49, 50, 51 and 52 are forward biased, current liows through thecapacitor 55 to charge the capacitor. The charge on the capacitor 55 isdependent upon the phase and amplitude of the signals from the.amplifier 12 relative to the phase and amplitude of the signals fromthe oscillator 10.

When the signal introduced to the phase detector 13 from the oscillator12 is in phase with the signal introduced to the phase detector from theamplifier 12, the signal at the junction of the diodes 49 and 50 has lapeak amplitude at the same time that the diodes in the phase detectorbecome forward biased by `the signals from the oscillator 10. Thiscauses signals of relatively high amplitude to be produced across thecapacitor 55, and may be seen in FGURE 2 by the signals to the left ofthe point 131 in curve 130.

When the signal introduced to the junction of the diodes 49 and 5t) fromthe amplifier 12 is 180 out of phase with the signal introduced to thephase detector from the oscillator 10, the signal at the junction of thediodes 49 and 50 has a relatively low amplitude at the time that thediodes in the phase detector become forward biased. This causes signalsto the right of the point 131 in FIGURE 2 to be produced.

The amplitude of the signals across the capacitor 55 varies only withthe amplitude of the signals introduced to the junction of the diodes 49and 50 from the amplifier 12 since the amplitude of the signals from theoscillator is constant. The resultant signal from the phase detector hasan amplitude representative of the deviation in tension in the tapemedium 4from the desired value. The position of the signal above orbelow the point 131 provides an indication of the direction of thatdeviation.

The parallel circuit composed of resistor 54 and capacitor 53 isinserted to compensate for phase shifts in the differential transformer11 and the amplifier 12. By providing this phase shift, the inputsignals to the phase detector 13 from the amplifier 12 are either inphase or 180 out of phase with the signals introduced to the phasedetector from the oscillator 10. The resistor 54 may have a value ofapproximately 100 kilohms and the capacitor 53 may have a value ofapproximately 0.1 microfarad.

The output from the phase detec-tor 13 constitutes a direct voltagewhich is impressed across the capacitor 55. This voltage is applied totransistors 56 and 57 in two emitter followers which are connected in acascade relationship. The transistors 56 and 57 can both be PNP type2N597. Connected from the emitter of transistor 56 to the referencepotential such as gro-und is a resistor 58, which may have a value of 57kilohms. Connected between the collector of transistor 56 and the sourceV1 is a resistor 59, which may have a value of ohms. Transistor 57 has aresistance @0 connected between the emitter and ground. The value of theresistance 60 may be 10 kilohms.

The emitter followers including the transistors 56 and 57 provideimpedance matching between the phase detector 13 and successive stages.The output of the emitter follower including the transistor 57 isproduced across the resistor 60 and applied to a lead-lag circuit in aseriesparallel arrangement. The lead circuit is composed of a parallelarrangement of a resistor d1 and a capacitor 62, which may respectivelyhave values of 88 kilohms and 2 icrofarads. The lag circuit is composedof a series arrangement of capacitor 653` and a rheostat 64, which mayrespectively have values of 50 microfarads and 10 irilohms. The leadcircuit of resistor 61 and capacitor 62 is used to compensate for thelag produced by the motor 304. The lag circuit of capacitor 63 andrheostat 64 is adjusted to compensate for phase shifts in other parts ofthe circuit.

The output from the lead-lag circuit is taken from the junction ofresistor 61 and capacitor 63 and is applied to a transistor 66 inanother emitter follower to provide addition-al impedance matching. Thetransistor o6 can be a PNP type 2N597. A resistance 67 which may have avalue of 100 ohms is connected between the collector of the transistor dand the source V1. Connected between the emitter of transistor e6 and asource V2 is a series circuit constituting a Zener diode 63 and aresistance 69. The source V2 can have a value of -{6 volts; the Zenerdiode 68 can be a type 1N758, and the resistance 69 can have a value of10 kilohrns. The Zener diode is used in the emitter follower output toprovide a constant voltage drop for matching voltage levels since thelevel of the output of the transistor d5 in an emitter follower isdiierent than the level of the next stage.

The output is ltaken from the junction of the Zener diod-e 68 andresistance 69 and applied to the' base of a transistor 70. Thetransistor 7? may be a PNP type 2N597. A resistance 71, which may have avalue of 390 ohms, is connected between the emitter of the transistor 70and the source V2, and a resistance 72, which may have a value of 1.5kilohms, is connected between the collector of the transistor and thesource V1.

The emitter of the transistor 70 is connected to the emitter of atransistor 73, which may also be a PNP type 2N597. Two resistors 74 and75, which may respectively have values of approximately 33() ohms and 1kilohm are in series between the sources V2 and V1 and have terminalscommon with the base of the transistor 73. A resistor 76 having a valueof approximately 1.5 kilohms is connected between the collector oftransistor 73 and the source V1. A feedback resistor 77 having a valueof approximately 10 lrilohms extends electrically between the collectorand the base of the transistor '73.

The output signals on the collectors of the transistors 70 and 73respectively pass through a pair of diodes 78 and 79 to the input of thenext stage. The collector of the transistor 73y is connected to thecathode of the diode 78, which may be a type 1N99. In like manner, thecollector of the transistor 70 is connected to the cathode of diode 79,which may also be a type D199. The anode of the diode- '78 is commonwith the anode of the diode 79. A resistor 80, which may have a value ofapproximately 22 kilohms, is disposed electrically between the junctionof the diode plates and the source V1. A capacitor 81, which may have avalue of approximately 0.22 microfarad, extends electrically from theplates of the diodes 7S and 79 to the reference potential such asground. The output signal from the plates of the diodes 78 and 79 is 9applied to the: base of' atransistor S2 in an emitter follower.

The transistors 76" an'd 73- and the diodes` 78 and 79 are included inthe differential amplifier 14-shownin FIG- URE 2. The' resistors 771,72, 74, 75 and76 provide the proper biases: andl loads andthe resistor77 provides a negative feedback for stabilizing the circuit. Thetransistors'70rand'73 operate as aA push-pull amplifier so that theoutput responsesof the' twotr-ansistorswill-A be cornplementary.ForeXample, when the signal. onthe baseof the transistor 70 tends toybecome'n'e'g'ative, an. increased current, flows through the transistor.This current produces4 an increased voltage acrosssthe` resistor 72= sothat the voltage on` the collector of the'transistor- 70 tends to becomepositive. The increased currentv also produces any increased voltagedrop-across the' resistor 71- so that the-voltage on they emittersfofthe transistors 70'and '73 tends to become negative. The vol-tage. on"the collector ofthe transistor 70tendsto become 'positive atthe sametime that the voltage'on the emitter of the transistor 7u tends tobecome negative because the transistor 7tlvhas a decreased'impedance asa'result of the increased ow of currentk through the transistor 70.

When the voltage on theV emitter of the'transistor 73 tends to becomenegative, the currentl through the transistor decreases. This causes thevoltage across the resistor 76, to decrease so that the voltage on thecollector of the transistor 73 tends to become negative. In this' way,the voltage on the collector of the transistor 70 tends to becomepositive at the same time that the voltage on the collector of thetransistor 73 tends to become negative. In like manner, a decreased flowof current through the transistor 70 tends to produce a negative voltageon the collector of the transistor '79 and a positive voltage on thecollector of the transistorY 73. lt will accordingly be seen that thetransistors 7i) and 73 provide, a differential effect with respect'tothe voltages on the collectors of the transistors. It is also seen thatthe transistors 79 and 73 serve asvalve means since they control thevflow of current in various stages of the differential amplifier 14'.

The signal atv the junction of the plates of diodes '723 andv79` followsthe voltage on the collector of one of the transistors 7i)V or 73,whichever has a relatively negative potential in comparison tothepotential on the other collector; This results in anoutput signal asshown in FG- URE 2 at Mii. For example, when the potential on thecollector. of the transistor 7t^is negative relative totlie potential onthe collectorr of the transistor 73,' thediode 79 -becomes conductivebefore the diode 78'so asto con'- trol the potential on the anodes oftheV diodes. Since the transistors 70' and 73 have a differentialoperation, one of the transistors controls the potentialon the plates ofthe diodes 7S and 79 on one side of the controlv point 131 andthe othertransistor controls the potential on the plates-of the diodes on theother side of the control point 131. fn this way', signals below thepoint 141 are eliminated since one or the otherof the diodes isv cut offdepend.- ing upon which collector has a relativelyv larger negativepotential. The curve 146 accordingly has signalsof equal amplituderepresentative of corresponding deviations in the tension in the'tape'mediunr on both sides of the desired value.

The output signal on the plates of the diodes 78' and 79 is applied tothe base of a transistor SZ'in an emitter follower. The transistor 82may be a PNP type 2N597. Connected between the collector of thetransistor 82 and .the source V1 is a resistor 83 which may have a valueof 100 ohms. A connection is made from the emitter of the transistor 83to the plate of a Zener'diode 84, preferably of a type 1N750. Thecathode ofthe Zener diode 84 hasa common connection with one terminal ofa resistor 85, which may have a value ofapproximately 8'60 ohms. Theother terminal of the resistor 85 is connected tothe base of atransistor 86. The transistor 86 may be a type 2N597.

Also connected to the base ofthe transistor 86 is one terminal of aresistor S7, which may have a value 0fv l kilohm. r[he other terminal ofthe resistor 87 is connected to the source V2. A feedback resistor 88preferably having a value of approximately 10 kilohrns extendselectrically from the base to the collector' of the transistor 86. Avariableresistor 39 preferably having avalue of approximately 390 ohmsis disposed electrically between the emitter of.A the transistor 86 andthe source V2. A resistorl 90is connected between the collector oftransistor' 86= and the` source V1 and is preferablyV provided with-avalue in the` orderl ofy lOO-ohms.

The output on the collector of the: transistor 85 is applied to'the baseof a transistor 91 in an emitter fol lower. The` transistor 91.can beaftype 2N597. A resistor 92, which canV have a value of 4.7 kilohms, isdisposedtelectrically between the emitter of transistor 91 and thesource V2. A- Zener diode 93 of type 1N75O and a resistor 9d having avalue of 10() ohms are in series between the collector of the transistor91 and the source V1.

The output on the emitter of transistor 91 is applied ,tothebasecfa-transistor 95 which may be a type 2N386.

A resistor 96, which may have avalue of l0 ohms, is disposed between thecollector of transistor 95 and a source V3 having a value preferably inthe order of 2S volts. Thevemitter of transistor 95 is connected tothesource V2 and also to the Ibases of transistors 97 and 9Sdisposed'electricaliy in parallel. Transistors 97 and 98 may both be oftype 2N`274. The emitters of. transistors 97 land 93 are connectedrespectively through resistors 99 and 161. to the reference potentialsuch as ground. Each resistor may have a value of 0.5 ohm. The col-vlectors of transistors 97 and 98 have a common connection to produce aresultant output control signal which is applied through reversingswitch 162 to one side of the motor 364: The other side of the motor isconnected through the reversing switch 102 to the source V3.

The operation of the transistorl 82A in an emitter fol lowerris similarto the transistor' 66 in an emitter follower since there is a Zenerdiod-e in the emitter lead to match a difference in voltage levels inthevemitter follower and thel neXtstag-e.r Resistors and183 provide theproper base and collector' bias for `the transistor `32. The capacitor81 by-passes alternating current to the reference potential from theinput signal applied to the base ofthe transistor 82. The output istaken from the junction of resistorsand 87 and applied `to the base of atransistor 86.

Transistor 86 hasV a gain of about 2, and inverts` the phase oftheincoming signal.4 A potentiometer S9 is used to vary the bias on theemitter of transistor Se. The bias is adjusted, as illustrated in FIG.2, sol that the output stages of the amplifierV cuts off at allmagnitude signals below the vertexv 141 of the control signal 140. Thisproduces an. out-put signal 150 from the direct current amplifierl whichhas its vertexV point 151 at the reference potential.

Feedback resistor SS provides stability for transistor 36,- and theoutput-signal from the transistor 86 is taken across load resistor 90.The output signal is then applied to two emitter followers in series.The first emitter follower has a transistor 91v and is used to match theimpedance of the transistor 86 to the following stages. The collectorbias supply of the transistor 91 hasa resistor 94 and a Zener diode 93to provide voltage stabilization. The output from the first emitterfollower is taken acrossran emitter load resistance 92 and is thenapplied-to the base of a transistor 95.

The second'emitter follower, which uses the transistor 95, provides bothimpedance matching and a current gain. The output of the transistor iscoupled electrically to the common connection of the `base of outputtransistor 97 and output transistor 93. The output transistors conduct ahigh current since they carry the control In' this way; the point 18i1frepresents the desiredtension inv the tape'30tl andfcorrespondstoy thepoint 13d' in the curve l'of'iFlGURE 2. The portion of the curve 180tothe" left of the point 18i1 represents deviationsin the tensionin thetape Stin a`v firstdirection from the desired"val`ue, and-"-the portionof the curve 1-80 to the right of; the point'utrltI representsdeviations-in the tension in the tape-in an oppositedirectionfromthe-'desired value. FIGURE; showsy in block formthecomplete system using; the photosensitive detectorf'199;

The'v output of the photosensitive detector 199 is applied to thedifferential amplifier 14 and the relay amplixlier'16, both olf` whichare also shown in'FIGURE 2. The differentialamplifiertransposesportionsl of the curve 18d below the point 1'81 to acorresponding position above the-point L31. This causes the curve lii'tobe produced andt-o beprovided withla'V- characteristic having its vertexpoint 141 corresponding to the point 1811i of the curveV 1580.

The output from the differential amplifier is` applied to the directcurrent amplifier 15 which ampliiies the signal andpassesonlysignalshaving a-magnitude above the point' 1-4'1 of* the curve 140i This'causes the curve 150 tobeproduced where the vertex` pointv has lbeenshifted to al reference potential such as ground. The output from the'direct current amplifier 15 is applied through the reversingA relay '17Ato control the speed ofthe motor 364; The reversing relay 17 iscontrolled by the relay amplifier 16 to be in either one of two statesofoperation. When the values of the signall 18d are above point 181, therelayamplifier 116 controls the reversing relay in its' first state ofoperation to apply signalsy of Va first polarity to the motor S64.' Whenthe input signals to the relay amplifier are below .point 1&1', therelay amplifier 16 controls the reversing relay in its second state ofoperation to apply signals of an opposite polarity to the m'otor 3G14.

As' will be seen, the embodiment of FIGURES 5, 6, 7 and 8 eliminatesmanyof the components necessary in the embodiment of FIGURES l, 2, 3 andy 4.However, since the system uses light to control the photosensitivedetector, care must be taken to prevent outside light from disruptingthe operation of the system fby adding spurious influences. This mayrequire that the light source 202, the photocell 201i' and 2011 and thevane 203 be disposed withinV a dar-k enclosure.

Although this application has 'been disclosed and illustrated withreference' to particular applications, the principles involved aresusceptible of numerous other applications which will'be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims;

What is claimed is: 1'. In combinati-on foruse with a movable medium, acontrol system for maintaining the tension in the movable medium at adesired value, including,

control means operatively coupled to the medium for exerting a drivingforce on the medium in dirst and second opposite directions' inaccordance with the direction and the degree o-f` excitation of thecontrol means, means responsive to the movement'of'the medium forproducing control signals having an amplitude on one' side of aparticular value yfor tensions in the movalble medium in excess of thedesired value and having an amplitude on the other side of theparticular value for tensions lower than the desired value,

switching means having first and second states of operationandfresponsive to the control signals to become operative in the firststate for control signals with an amplitude on one side of the saidparticular value. and to become operative in the second state ofoperation for. control signals with an amplitude on the other side ofthe said particular value,

i4 means responsive tothe control=signalsfor transposing control signalson one sideof the particular value to acorrespondingrposition ontheother side of the particular value tov obtain signals. of equalamplitude for corresponding deviationsiin tensions on bothssides of thedesired value, and

means' responsive to changes in.l the'states of operation of theswitching means-betweenthe-*firstl and second states to change thedirection-,of excitation; ofthe control means andresp'onsive: toV thetransposed control signal to change the. degree' ofi excitation of thecontrol means in accordance lwiththe amplitude of the transposed controlsignal.y

2.' in combination for use with amovable recording medium, a motorcontrol systemfor maintaining the tension in' the medium at a=desired`value, including,

control means operatively coupled toV the movable recording mediu-m forAexerting a force on the medium in first and second opposite directionsand. with a variableintensity to control the' tensionin the medium,

means responsive tothe movement of` the medium for producing a control'signal having a phase` correspon'ding to'the direction of thede-'viationof the tension in the movable recording medium from the. de# sired valueand having an amplitude corresponding to the magnitude off deviation ofthe tension in the movable recording medium from' the desired value,means operatively co-upled'to the control signal means for biasing thecontrol-signalI to' the same polarity on opposite sides of a null pointcorresponding to the value of the biasing means and regardless of thedirection of any deviation of the tension in the movable recordingmedium from the desired value, reversing means having first and secondstates of operation and responsive to the control signal to becomeoperative in the first state of values ofthe' control signal on one sideof' the null point and to become operativein the second state lforvalues of the control signal on the other side of the null point, meanstransposingthe 'biased control signal on one side olf the null point toa corresponding value` on the other side of the nullV point to have allvaluesof the control signal on the same side of the null point, meansresponsive to the amplitude-of the transposed control signal to regulatethe intensity of the force exerted lby said control means on saidmovable recording medium, and means responsive to the operation of saidreversing means in the first and second states to regulate the directionof the force exerted Iby said control means on said movable recordingmedium.

3. -In combination for use with a movable medium, a control system form-aintainingthe tension in the movable medium at a desired value,including control means operatively coupled to the medium for exerting adriving force on the medium in first and second opposite directions inaccordance with the direction and the degree of excitationof the controlmeans,

means responsive to the movements of the medium for producing controlsignals having an amplitude on one side ofa particular value fortensions in the movable medium in excess of the desired value and havingan amplitude on the other side of the particular value for tensionslower than the desired value,

switching means having first and second states of operation andresponsive to the control signals to become operative in the rst statefor control signals with -an amplitude on one side of the particularvalue and to become operative in the second state for control signalswith an amplitude on the other side of the particular value,

transposing means including iirst and second valve -means interconnectedto produce complementary impedance characteristics for the first andsecond valve means in accordance with the introduction of the directionof rotation of the motor means in accordance with the state of operationof the reversing means.

5. In combination for use with a movable recording direct current meansoperatively coupled to the control' signal means for biasing the controlsignal to the same polarity on opposite sides of a particular value andto the same value for corresponding devisignals to the iirst Valve meansand responsive to ations in the tension in the movable recordingVariatlOnS lll th@ Characteristics Of th@ Control Slgmedum in directions,from the desired Value, nals t0 obtain irst variations in impedance inthe switching means having irst and second states of opiirst valve meansand opposite variations in impedance eration and responsive to thebiased control signal in the second valve means, to become operative inthe first state of operation for tlie transposing means furtherincluding means intercontrol signals to one side of the particular valueand connected to the iirst and second valve means and to beeorneoperative in the Second Stateof operation responsive to the impedancecharacteristics of the for control signals to the other side of theparticular iirst and second valve means to produce the same Valuesamplitude output signals for corresponding deviameans responsive to theamplitude of the biased contions in the tension in the medium on bothsides of trol signals on one side 0f the particular value to the desiredvalue, and control the force exerted by the direct current motor meansresponsive to changes in the states of operation means in one directionand responsive to the ampliof the Switching means between the first andsecond tude of the biased control signals on the other side of states tochange the direction of excitation ot the the particular value tocontrol the force exerted by control means and responsive to the outputsignals the direct current motor means in the other direcfrom thetransposing means to change the degree of tion with a resultantregulation of the tension in excitation of the control means inaccordance with the movable recording medium, and tile amplitude of theoutput signalmeans responsive to the lirst and second states of op- 4'.In COInbinatiOn With a movable tape medium, a con' eration of theswitching means to control the polarity trol system for maintaining thetension in the medium at of the biased Control signals applied to themotor n desired value, including, means to change the direction of theforce applied means for Producing signals having n single polarity bythe motor means on the movable recording and having a slopingcharacteristic passing through niedinrn, a particular valuerepresentative of tile desired ten 6. In combination for use with amovable medium, a sion in tlie tape medium and having values of thecontrol system for maintaining the tension in the movable signal locatedon the sloping characteristic above and medium at a desired vaine,including, 'below the Particular value in representation of devi controlmeans operatively coupled to the medium for ations in the tension intl'ie tape medium in both exerting a driving force on the medium infirst and directions from the desired value second opposite directionsin accordance with the reversing means having iirst and second states ofop" direction and the degree of excitation of the control eration andresponsive to the signal having the means, sloping characteristic toobtain an operation of the means responsive to the movement of themedium for reversing means in the iirst state of operation for producingcontrol signals having an amplitude on signals above the Particularvelue and in the Second one side of a particular value for tensions inthe state ot operation for signals beloW the Particular 40 movablemedium in excess of the desired value and value, having an amplitude onthe other side of the parmeans responsive to the signal having thesloping chartisular v`7nine for lrensions iower than the desiredacteristic for transposing the sloping characteristic Value, to asubstantially V characteristics With the vertex relay control meansresponsive to the control signals of the V representative of the desiredtension m the and including a rst and Second Vaive rneans inter.. tapemedium, connected in a cascade arrangement and having the meansresponsive to the signal having the V charac' rst valve means in a stateof saturation with the teristic for biasing the signal to a positionWith the introduction of control Signals to one side of the vertex ofthe V characteristic at areference potential, particular Value to biasthe second Vsive rneans ro and a state of non-conduction and having thelirst valve motor means operatively coupled to the reversing means meansin a state of non-conduction With the introand responsive to the biasedSignal and operatively duction of control signals to the other side ofthe coupled to the movable medium for regulating the particular value tobias the second valve means to speed of the motor means in accordancewith the n state of conduction, amplitude of the biased Signal and forregulating relay means having a iirst and second state of operation andresponsive to the second valve means of the relay control means toobtain the operation of the relay means in the first state of operationwhen the second valve means are in a state of non-conmedium, a directcurrent system for maintaining the ten- G0 sion in the movable recordingmedium at a desired value, including,

duction and to obtain the operation of the relay means in a second stateof operation when the second valve means are in a state of conduction,and

direct current motor means operatively coupled to the medium to exert aforce on the medium in lirst and means responsive to the amplitude ofthe control signals on one side of the particular value and to the Scmldappositi.: dilecticms for Controlling the. ten" G5 relay means in afirst state of operation to control sion inthe inedmm 1n accordance withthe amplitude the force exnrted by the control mams on the me gld polamyQi a direct current Signal applied t0 dium in one direction andresponsive tothe amplit e motor 1 I means operatively coupled to .themedium for Proudh on lme other 5.1 dethof the lartcllmfvlue End ducmgd1rect current control signals having an ampli- 70 O e le ay means m esecon s a e o Pera lon tude in accordance with the deviation in tensionin the movable recording medium from the desired value and having apolarity in accordance with the direction of deviation of the tension inthe movable recording medium from the desired value,

to control the force exerted by the control means on the medium in theother direction. 7. In combination for use with a movable medium, a

control system for maintaining the tension in the movable medium at adesired value, including,

actresses control means operatively coupled to the movable medium fiorapplying a force -to the medium in first and second directions and witha variable intensity to maintain the tension in the movable medium atthe desired value,

phase detector means responsive to the output signal from thedifferential transformer and to the' reference signal from theoscillator means for producing a control signal having values on oneyside of a particular value for tensions in excess of the dea source ofenergy, sired value and having Values on the other side of Vfirst andsecond pickup means responsive to the enthe particular value fortensions lower than the ergy from the source and differentiallyintercondesired value, nected to produce first and second outpu-tsignals relay control means having a state of conduction and withcomplementary characteristics in accordance a state of non-conductionand response to the conwith the amount of energy from the source and totrol signal to become operative in the state of conproduce a resultantoutput signal in accordance duction for control signals with anamplitude on with the relative characteristics of the first and one sideof the particular value and to become opsecond output signals, erativein the state of non-conduction for control control member operativelycoupled to the medium signals with an amplitude 'on the other side ofthe and disposed between the source and the first and particular value,second pickup means for displacements from a rnerelay means having afirst and second state of operadian position in accor-dance with thedirection and tion and responsive to the relay control means tomagnitude of the deviation in the tension in the become operative in thefirst state of operation with medium from the desired value, V2() therelay control means in a state of conduction regulating means responsiveto the tension in the movand to become operative in the second state ofopable medium and including the control member to eration with the relaycontrol mea-ns in a state of control the coupling of the energy from thesource to non-conduction, the first and second pickup means for theproducmeans responsive to the control signals for transposing tion ofthe complementary signals by the first and control signals on one sideof the particular value second pickup means and for the production of anto a corresponding position on the other side ofthe output signal havinga particular value at the departicular value to obtain aV-characteristic signal sired tension of the medium, and havingamplitudes having signals of equal amplitude for corresponding onopposite sides of the particular value for respecdeviations in thetension in the medium on both sides tive deviations in the tension inthe medium above of .the desired value, and below the desired value,means responsive to the signal having a V characterswitching meansresponsive to the resultant output istic for biasing the signal to aposition with the signal from the first and second pickup means tobevertex of lthe V characteristic at a reference pocome opera-tive in afirst state for values of the tential, and resultant output Signal 0n011e Side 0f the paltClllar 35 means responsive to changes in the statesof operation value and to become operative in a second state of therelay means between the first and second for values of the resultantoutput signal on the states to change the direction of excitation of the.other side ofthe particular value, and con-trol means and responsive to'the biased V- means responsive lto the amplitude of the resultantcharacteristic signal to change the degree of excitaoutput signal fromthe first and second pickup means 40 tion of the control means irlaccordance with the to control the intensity of the force applied by theamplitude of the biased V-characteristic signal. Control means t0 lh@IDOVable medium and TCSPOH- 9. In combination for use with a movablemedium, a sive to the opratiOD 0f 'the Switching means in the controlsystem for maintaining the tension in the moviirst and second states tocontrol the direction of able medium atadesired value, including, theOTC@ applied t0 the mOVHlDle medium by the 45 control means operativelycoupled to the medium for control means. exerting a driving force on themedium in first and 3- In Combination fOr USe With a mOVEL-bl medium, asecond opposite directions in accordance with the COIltrOl SYStem formaintaining the tSDSiOIl iIl the, Inovdirection and the degree ofexcitation of the control able medium at a desired value, including,means,

ycontrol means operatively coupled tothe movable mephotosensitive meansresponsive to the tension in the dium for exerting a force on the mediumin first movable medium, including, and second opposite directions andwith a variable a light source for producing a beam of light, first andAintensity to control the tension in the medium, second photosecsitivecell means connected in series oscillator means for producing areference signal at and responsive to the light from the source toproduce a particular frequency, a control signal at the junction of thephotosensitive differential transformer means, including, cell means, aninput-winding electrically connected tothe oscillator regulating vanemeans interposed between the light means to prod-,ucc a signal at theparticular frequency, source and `the tirst and second photosensitivecell two output windings interconnected in an opposite means to controlthe amount of light passing lfrom sense and electrically coupled to theinput winding G0 the light source to .each of the first and secondphototo produce an output signal from the two output sensitive cellmeans and responsive to changes in the windings, tension in the medium.to pass an increased amount regulating `means disposed in magneticallycoupled relaof light from the light source to one photosensitivetionship to the input winding and the two output cell means and acorrespondingly decreased amount windings and responsive to the changesin the moveof light from the light Asource :to the other photosensimentof the medium from the desired value for tive cell means for theproduction at the junction of regulating the coupling of the inputwinding to the the photocell means of a control signal having valuesoutput winding in accordance with the direction and on one side of aparticular value for tensions in exmagniture of ldeviation in thetension in the mecess of the desired tension in the medium and havingdium from the desired value to obtain the production values on the otherside of theV particular value for by the output winding of a first phasefor the outtensions in the medium lower than the desired' tenput signalfor tensions in the medium on one side sion, of the desired value and ofa second opposite phase relay control -means having a state ofconduction and a for the output signal for tensions on the other stateof non-conduction and responsive tothe control side of the desiredvalue, signal to become operative in the state of conduction for controlsignals to one side of the particular value and to become operative inthe state of nonconduction for control signals to the other side of theparticular value,

relay means having a first and second state of operation Iandoperatively coupled to the relay control means to become operative inthe first state with the relay control means in a state of conductionand to become operative in the second state of operation with the relaycontrol means in a state of non-conduction,

means responsive to the control signals for transposing control signalson one side of the particular value to a corresponding position on theother side of the particular value to obtain signals of equal amplitudefor corresponding deviations in the tension in the medium on both sidesof the desired value, and

means responsive to changes in the states of operation of the relaymeans between the first and second states :to change the direction ofexcitation of the control means and responsive to the transposed controlsignals to change ythe degree of excitation of the control means inaccordance with the amplitude of the transposed control signal.

10. In combination for use with a movable medium, a

control system for maintaining the tension in the movable medium at adesired value, including,

control means operatively coupled to the medium for exerting a drivingforce on the medium in first and second opposite directions inaccordance with the direction and the degree of excitation of thecontrol means,

oscillator means for producing a constant amplitude signal having aparticular phase relationship,

differential transformer means responsive to the oscillator means and tothe tension in the movable rnedium for producing an output signal havingan amplitude in accordance with the tension in the movable medium andhaving an in-phase relationship with the signal of the oscillator meanswhen the tension in the movable medium is to one side of the desiredtension and an out-of-phase relationship with the signal of theoscillator means when the tension in the movable medium is to the otherside of the desired tension,

phase detector means responsive to the output signal from thedifferential transformer and to the signal from the oscillator means forproducing a control signal having values on one side of a particularvalue for an in-phase relationship between the signals from theoscillator means and the differential transformer means and havingvalues of the control signal on the other side ofthe particular valuefor an outofphase relationship between the signals from the oscillatormeans and the differential transformer means,

relay control means having a state of conduction and a state ofnon-conduction and responsive to thc control signal of the phasedetector means to become operative in the state of conduction forcontrol signals to one side of the particular value and to becomeoperative in the state of non-conduction for control signals to theother side of the particular value,

relay means having a first and second state of operation and responsiveto the relay control means to become operative in the first state ofoperation with the relay control means in a state of conduction and tobecome operative in the second state of operation with the relay controlmeans in a state of nonconduction,

means responsive to the control signals from the phase detector meansfor transposing the control signals on one side of the particular valueto a corresponding position on the oth-er side of the particular valuelto obtain a Vcharacteristic output signal having signals of equalamplitude for corresponding deviations in tension on both sides of thedesired value,

2t), means responsive tothe signal having a V characteristic for biasingthe signal to a position with the vertex of the V characteristic at areference potential, and means responsive to changes in the states ofoperation of the relay means between the first and second states tochange the direction of excitation of the control means and responsiveto the biased V-characteristic signal to change the degree of excitationof the control means in accordance with the amplitude of the transposedoutput signal. 11. In combination ttor use with a movable medium, acontrol syste-m for maintaining the tension in the movable medium at adesired value, including,

control means operatively coupled to the medium for exerting a drivingforce on the medium in iirst and second opposite directions in`accordance with the direction and the -degree or excitation of thecontrol means, photosensitive means responsive to the tension in themovable lmedium for producing a direct current output signal having asingle polarity and having an amplitude to one side o-f a particularvalue for tensions in the movable medium in excess of the desired valueand having an amplitude to the other side of a particular value lfortension in the movable mediurn lower than the desired value, relaycontrol means having a state of conduction and a state of non-conduction.and responsive to the output signal of the p-hotosensitive means tobecome operative in the state of conduction for output signals with anamplitude to one side of the `particular value `and to become operativein t-he state of non-cond=uc tion for output signals with an amplitudeto the other side of the particular value, relay means .having a rst andsecond state of operation and responsive to the relay control means tobecome operative in the iirst state of operation with'. the relaycontrol means in la state of conduction Iand to become operative in thesecond state of operation with the relay control means in a state ofnon-conduction, means responsive to the output signals from thephotosensitive means for tnansposing the output signals on one side ofthe particular value to .a corresponding position on the other side ofthe particular value to obtain a V-characteristic output signal havingsignals of equal amplitude for corresponding deviations in tension onboth sides of the desired value, means responsive to the signal having aV characteristic for biasing the signal to a position with the vertex ofthe V characteristic at a reference potential, and means responsive tochanges in the states of operation of the relay means between the firstand second states to change the direction of excitation of the controlmeans and responsive to the biased V-characteristic signal to change thedegree of excitation of the control means lin accordance with theamplitude of the transposed output signal. 12. A control system asdefined in claim 3 wherein the first and second valve means of thetransposing means are yfirst and second PNP transistors having a commonconnection between the emitters and Cnaving the control signalintroduce-d at the base of the iirst transistor and the signal at thebase of the second transistor interrelated with the current flowing inthe rst transistor to produce the complementary impedancecharacteristics at the collectors of the first and second transistors,and the means respon sive to the impedance characteristics of the firstand second valve means are two diodes having their cathodes connectedrespectively to the collectors of the `first and second transistors yandYhaving a common connection between their anodes to produce the outputsignal at the common connection.

13. The control signal as defined in claim 6 wherein t-he -rst andsecond valve means of the relay control means are rst and second PNPtransistors having the control signals applied to the base of the rsttransistor with the input to the base of the second transistor takenfrom the collector of the iirst transistor and dependent upon thecur-rent flowing through the iirst transistor to control Jche currentflowing through the second transistor to control the lrelay means.

14. The combination of claim 4 wherein the means for producing thesignal 4having a sloping characteristic includes a light responsiveelement.

15. The combination of claim 4 wherein the means for producing thesignal having a sloping characteristic includes rst and secondphotosensitive means interconnected in a differential arrangememnt.

16. In combination `for |use with a movable medi-um, a cont-rol systemfor maintaining the tension in the movable medium at a desired value,includ-ing,

control means operatively coupled to the medium for exerting a drivingforce on the movable medium in first and second opposite directions inaccordance With the direction and the degree of excitation of thecontrol means,

oscillator means for producing a constant amplitude signal having aparticular phase relationship, differential transformer means responsiveto the oscillator means and to the tension in the movable medium forproducing an output signal having an amplitude in accordance with thetension in the lmovable medium and thaving an in-phase relationship withthe signal of the oscillator means when the tension in the movablemedium is to one side of the desi-red value and an out-of-phaserelationship with the sig- Inal of the oscillator means when the tensionin the movable medium is to the other side of the desired value,

phase detector means including four diodes in a bridge circuitresponsive to the output signal from the diiferential transformer and tothe signal from the oscillator means ytor producing a control signalhaving values on one side of a particular value for an inphaserelationship between the signals -from the oscillator means and thedifferential transformer means and having values of the control signalon the other side of the particular value for an out-of-phaserelationship between the signals from the oscillator means and thedifferential transformer means, and

means responsive to the control signals on one side of the particularvalue to control the `force exerted by the control means on the movablemeans in the rst direction and responsive to the control signals on theother side of the particular value to cont-rol the force exerted by thecontrol means in the second opposite direction.

References Cited bythe Examiner UNITED STATES PATENTS 1,969,536 8/1934Winne.

2,586,076 2/1952 Nichols 318-6 2,977,517 3/1961 Baybick 318-7 2,985,3965/1961 Johnson S18-7 X 3,092,764 6/1963 Cooper 318-6 ORS L. RADER,Primary Examiner.

T. LYNCH, Assistant Examiner.

1. IN COMBINATION FOR USE WITH A MOVABLE MEDIUM, A CONTROL SYSTEM FORMAINTAINING THE TENSION IN THE MOVABLE MEDIUM AT A DESIRED VALUE,INCLUDING, CONTROL MEANS OPERATIVELY COUPLED TO THE MEDIUM FOR EXERTINGA DRIVING FORCE ON THE MEDIUM IN FIRST AND SECOND OPPOSITE DIRECTIONS INAC CORDANCE WITH THE DIRECTION AND THE DEGREE OF EXCITATION OF THECONTROL MEANS, MEANS RESPONSIVE TO THE MOVEMENT OF THE MEDIUM FORPRODUCING CONTROL SIGNALS HAVING AN AMPLITUDE ON ONE SIDE OF APARTICULAR VALUE FOR TENSIONS IN THE MOVABLE MEDIUM IN EXCESS OF THEDESIRED VALUE AND HVING AN AMPLITUDE ON THE OTHER SIDE OF THE PARTICULARVALUE FOR TENSIONS LOWER THAN THE DESIRED VALUE, SWITCHING MEANS HAVINGFIRST AND SECOND STATES OF OPERATION AND RESPONSIVE TO THE CONTROLSIGNALS TO BECOME OPERATIVE IN THE FIRST STATE FOR CONTROL SIGNALS WITHAN AMPLITUDE ON ONE SIDE OF THE SAID PARTICULAR VALUE AND TO BECOMEOPERATIVE IN THE SECOND STATE OF OPERATION FOR CONTROL SIGNALS WITH ANAMPLITUDE ON THE OTHER SIDE OF THE SAID PARTICULAR VALUE, MEANSRESPONSIVE TO THE CONTROL SIGNALS FOR TRANSPOSING CONTROL SIGNALS ON ONESIDE OF THE PARTICULAR VALUE TO A CORRESPONDING POSITION ON THE OTHERSIDE OF THE PARTICULAR VALUE TO OBTAIN SIGNALS OF EQUAL AMPLITUDE FORCORRESPONDING DEVIATIONS IN TENSIONS ON BOTH SIDES OF THE DESIRED VALUE,AND MEANS RESPONSIVE TO CHANGES IN THE STATES OF OPERATION OF THESWITCHING MEANS BETWEEN THE FIRST AND SECOND STATES TO CHANGES THEDIRECTION OF EXCITATION OF THE CONTROL MEANS AND RESPONSIVE TO THETRANSPOSED CONTORL SIGNAL TO CHANGE THE DEGREE OF EXCITATION OF THECONTROL MEANS IN ACCORDANCE WITH TEH AMPLITUDE OF THE TRANSPOSED CONTROLSIGNAL.